Low velocity oxygen-fueled flame spray method and apparatus for making ferrite material products and products produced thereby

ABSTRACT

A method for making ferrite powder may include providing ferrite feed materials in a form of particles, such as having different sizes and irregular shapes. The method may further include exposing the ferrite feed materials to a low velocity oxygen-fueled (LVOF) flame spray. This may provide a more spherical shape to irregularly shaped particles to thereby make the ferrite powder. An apparatus for making ferrite powder may include a feeder for ferrite feed materials and a LVOF flame spray system for exposing the ferrite feed materials to the flame spray.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/774,309 filed on Feb. 17, 2006, the entiredisclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of ferrite products and, morespecifically, to the field of production of ferrite products.

BACKGROUND OF THE INVENTION

Mixtures of iron oxide, oxides of transition metals, other metals, andsemi-metals are basic components in the manufacture of many productsthat are used for electromagnetic interference (EMI) suppressionfilters, inductors, and reprographic system components, among others.Ferrite materials, commonly called spinel, may be produced by formingsmall particles of chemical oxides. Other crystal structures, such asgarnet, and hexagonal ferrite, may be also be produced. Such spinelferrites may be based on an iron oxide denoted by the formula, MeFe₂O₄,and may contain as the Me element, for example, some combination ofsubstituted transition metals such as manganese (Mn), nickel (Ni),cobalt (Co), vanadium (V), as well as other oxides of metals such asmagnesium (Mg), copper (Cu), aluminum (Al), strontium (Sr), and zinc(Zn).

These ferrites may also contain semi-metals such as silicon (Si), andother additives such as titanium (Ti), tantalum (Ta), niobium (Nb),vanadium (V), and even alkaline earth metals such as calcium (Ca). Insome cases, alkali metals may reside in the ferrite or in other phases.

Although referred to as spinel, garnet and hexagonal ferrite, thesematerials may be complex multiphase materials containing phases such asFeO and glass formers that are used to control bulk electricalresistivity, eddy currents, frequency response of material impedance,magnetic hysteretic characteristics, total magnetic moment, andsintering characteristics.

The production of ferrites may be controlled intentionally to producespinels of oxides of iron and other elements, which may occur in morethan one valence state having controlled ferrimagnetic properties. Inmany applications a particular surface morphology and a specific size ofpowder may be required to achieve desired product properties.

To obtain these desired product properties in a final product shape, aferrite, or precursor mixture, must first be produced. This may beaccomplished by methods well known to persons experienced in thetechnology and may include processes such as chemical precipitation, theuse of naturally occurring oxide ores, or conversion of aqueous solutionof metal salts such as chlorides, and even melting the startingingredients.

The overall elemental composition of the incipient ferrite spinel may becreated by mixing exact proportions of metal oxides, or chemicalprecursors. The overall elemental composition may require grindingelemental oxides in a proper proportion into an intimate mixture ofsmall particle size, and adjusting the composition of the final mixture,which may then be spray dried.

Such mixtures may be subjected to intermediate thermal treatments inrotary or fixed kilns to partially react them, or to produce particlesizes useful for subsequent mechanical processing. Afterthermal-mechanical processing, the materials may be ground again intosmall particle size, and the composition may be adjusted to meet atarget terminal composition. The materials may thereafter be spray driedin an aqueous process to create an aggregate that may be mixed andblended, pressed into a shape, and sintered.

For some applications, such as reprographic use, the powder may be usedin the spray dried and sintered form. In other applications, the powdermay be used with, or without, spray drying when it is intended as anadditive to a mixture of an inorganic or organic binder.

A process for producing a pre-reacted oxide powder is disclosed in U.S.Pat. No. 5,976,488 to Workman et al., the entire disclosure of which isincorporated herein by reference. While this process producespre-reacted powder, its particular phase mixture is not typicallysuitable for direct use in ferrite products without further processingand thermal treatments. Moreover, the atmospheric conditions used toproduce such powders may not be effectively controlled to achieve adesired oxidation state and phase composition of all the iron and otherelements.

The process described in the Workman et al. patent produces particlemorphology that is somewhat useful for carrier bead, but the ratio offerri-magnetic spinel to non-ferrimagnetic oxides of iron and otherelements leads to a magnetic moment that may be too low for use.Further, the ratio may also lead to a volume electrical resistivity thatis not suitable. Accordingly, this product must be further processed toproduce a useful carrier bead.

Additionally, the process described in the Workman et al. patent mayproduce detrimental particle shapes, such as broken particles with sharpedges and elongate particles. It may be difficult and costly to removethese irregular particles by traditional separation methods since theyexist in a range of sizes. Further, a significant percentage of usefulspherical powder may be lost when conventional separation methods areused. A negative economic impact is observed due to a small percentageof total powder that is usable.

A significant advance in the area of making ferrite powders is disclosedin U.S. Pat. No. 7,118,728 to Paris et al., assigned to the assignee ofthe present invention and the entire contents of which are incorporatedherein by reference. In particular, the patent discloses a method formaking ferrite powder by exposing ferrite feed materials to a plasma sothat the irregularly shaped particles are made more regular in shape. Inaddition, U.S. Pat. Nos. 5,462,686 and 6,793,842 also disclose variousapproaches to producing a ferrite powder using thermal processing.

Despite continual developments and improvements in the manufacture offerrite powders and articles, there still exists a need for such ferritematerials with enhanced properties, and there still exists a need formore efficient related manufacturing processes.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of thepresent invention to provide a method and associated apparatus formaking ferrite powder with enhanced properties and in an efficientmanner.

This and other objects, features and advantages in accordance with theinvention are provided by a method for making ferrite powder comprisingproviding ferrite feed materials in a form of particles, and exposingferrite feed materials in the form of particles to a low velocityoxygen-fueled (LVOF) flame spray to thereby make the ferrite powder. Theexposing may comprise axially introducing the ferrite feed materials inthe form of particles to the LVOF flame spray. In addition, the LVOFflame spray may operate at a temperature of less than about 5,000° C.,and with a flame velocity of less than about 1,000 feet per second, forexample.

The ferrite feed materials prior to exposing may comprise irregularlyshaped particles. The LVOF exposure may produce more spherically shapedparticles from the irregularly shaped particles. Exposing the ferritematerials to lower temperature environment of the LVOF flame spray mayenhance the economical efficiency of production of ferrite particles,and advantageously increase a yield of ferrite powder production.

The method may further include controlling at least one of providing andexposing to make the ferrite powder to have at least one of apredetermined phase ratio, surface morphology, density, magnetic moment,and volume electrical resistivity. For example, the controlling maycomprise controlling at least one of a feed rate, an exposure time, anda temperature of the ferrite feed materials during the exposing. Thecontrolling may also comprise controlling a composition of the ferritefeed materials. Moreover, controlling may comprise controllablysupplying at least one additional material to the ferrite feed materialsduring the exposing. For example, the at least one additional materialmay comprise at least one of oxygen, hydrogen, an inert gas, andcalcined ferrite feed materials. Controllably supplying may comprisecoating the ferrite feed materials with at least one of a silicate,alumina, and an organo-metallic.

The ferrite feed materials comprise at least one of nickel ferriteparticles, manganese ferrite particles, magnesium ferrite particles,strontium ferrite particles, and zinc ferrite particles. The ferritefeed materials may also comprise metal oxides. The method may furtherinclude sorting the ferrite powder to have particle sizes within apredetermined range.

Another method aspect is directed to making a ferrite article. Themethod may include providing ferrite feed materials in a form ofparticles, exposing ferrite feed materials in the form of particles to alow velocity oxygen-fueled (LVOF) flame spray to thereby make a ferritepowder, and forming the ferrite powder into a ferrite article. Theferrite articles may be carrier beads, an inert anode, or a body of aninductor, for example.

Yet another aspect of the invention is directed to an apparatus formaking ferrite powder comprising a feeder for ferrite feed materials ina form of particles, and an LVOF flame spray system for exposing theferrite feed materials to an LVOF flame spray to thereby make theferrite powder. The LVOF flame spray system may axially introduce theferrite feed materials in the form of particles to the LVOF flame spray.The LVOF flame spray system may generate the LVOF flame spray at atemperature of less than about 5,000° C., and at a flame velocity ofless than about 1,000 feet per second, for example. The apparatus mayfurther comprise a controller for controlling at least one of the feederand the LVOF flame spray system to make the ferrite powder to have atleast one of a predetermined phase ratio, surface morphology, density,magnetic moment, and volume electrical resistivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating a method for making ferrite powderaccording to the present invention.

FIG. 2 is a flow chart illustrating another method for making ferritepowder according to the present invention.

FIG. 3 is a flow chart illustrating yet another method for makingferrite powder according to the present invention.

FIG. 4 is a flow chart illustrating still another method for makingferrite powder according to the present invention.

FIG. 5 is a schematic block diagram of an apparatus for making ferritepowder according to the present invention.

FIG. 6 is Table 1 showing a summary of comparative trials.

FIG. 7 is Table 2 showing a comparison of powder properties.

FIG. 8 is Table 3 showing a comparison of costs.

FIG. 9 is a scanning electron microscope photograph of a ferrite powderproduced in accordance with a prior art nitrogen plasma spray.

FIG. 10 is a scanning electron microscope photograph of a ferrite powderproduced in accordance with a prior art argon plasma spray.

FIG. 11 is a scanning electron microscope photograph of a ferrite powderproduced in accordance with the LVOF spray process in accordance withthe present invention.

FIG. 12 is a scanning electron microscope photograph of a ferrite powderproduced in accordance with a prior art nitrogen plasma spray andillustrating an unmelted particle.

FIG. 13A is a flowchart for an embodiment for making ferrite poweraccording to the invention.

FIG. 13B is a flowchart for an embodiment for making ferrite poweraccording to the prior art plasma spray process.

FIG. 13C is a flowchart for another embodiment for making ferrite poweraccording to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

Referring initially to the flow chart 20 of FIG. 1, a method for makingferrite powder is now described. From the start (Block 22) ferrite feedmaterials are provided at Block 24. The composition of the ferrite feedmaterial may be controlled when the ferrite feed material is provided atBlock 24.

The ferrite feed materials may be in the form of particles havingdifferent sizes and irregular shapes. The ferrite feed materials may,for example, comprise nickel ferrite particles or zinc ferriteparticles. The ferrite feed materials may also comprise manganeseferrite particles or magnesium ferrite particles so that the ferritefeed materials are advantageously environmentally friendly. Thoseskilled in the art will appreciate that the ferrite feed materials maycomprise any combination of manganese ferrite particles and magnesiumferrite particles. Those skilled in the art will also appreciate thatthe ferrite feed materials may, of course, comprise any other ferriteparticles.

At Block 26, the ferrite feed materials are subjected or exposed to anLVOF flame spray. The exposure may, for example, be in the form ofpassage through an LVOF flame spray gun as will be understood by thoseskilled in the art. Other devices may also provide the LVOF flame sprayexposure. The exposure of the ferrite feed materials to the LVOF flamespray advantageously provides a more spherical shape to irregularlyshaped particles to thereby make the ferrite powder. More specifically,the irregularly shaped particles are melted into more spherically shapedparticles. Making the particles more spherically shaped advantageouslyenhances the efficiency and economics of ferrite particle production.Further, exposure of the ferrite feed materials to the LVOF flame sprayadvantageously enhances surface characteristics of the ferrite powder.The ferrite powder may be made to have a predetermined phase ratio,surface morphology, density, magnetic moment, and/or volume electricalresistivity while maximizing yield and minimizing oxidation residue, forexample.

A number of different variables of the ferrite powder production may becontrolled during the exposure of the ferrite to the LVOF flame spray.For example, the feed rate of the ferrite feed material may becontrolled, the exposure time of the ferrite feed material to the LVOFflame spray may be controlled, and the temperature of the ferrite feedmaterial during exposure to the LVOF combustion-heated gas may becontrolled at Block 28.

Further, a rate of heat transfer from the LVOF flame spray may becontrolled to thereby control the temperature of the ferrite feedmaterial. The volume of combustion gas, as well as the type ofcombustion and oxidation gasses, used during LVOF flame spray exposuremay also be controlled. For example, the flow rate of the carrier gasmay be regulated. The flow of combustion and oxidation gasses may alsobe controlled, as understood by those skilled in the art.

Additional materials may be controllably supplied to the ferrite feedmaterials during exposure to the LVOF flame spray at Block 30. Theadditional materials may, for example, include oxygen, hydrogen, inertgas, or any other suitable material, as understood by those skilled inthe art.

The ferrite powder generally comprises different sizes. Accordingly, atBlock 32, the ferrite powder is sorted to have particles sized within apredetermined range. At Block 34, the ferrite powder may be formed intoa ferrite article, before stopping at Block 36. More specifically, theferrite powder may be formed into carrier beads, an inert anode, or abody of an inductor, for example. Those having skill in the art willrecognize that the ferrite powder may also be formed into any number ofother articles.

Referring now additionally to the flow chart 40 of FIG. 2, anothermethod for making ferrite powder is now described. From the start (Block42), the ferrite feed materials are provided at Block 44. At Block 46the ferrite feed materials are calcined. The ferrite feed materials arepreferably provided by either spray dried powder that has been calcined,or uncalcined spray dried powder. This enhances densification of theferrite feed material and further enhances the economic efficiency ofproduction of the ferrite powder.

The ferrite feed materials may also be coated. The ferrite feedmaterials are preferably coated with a non-ferrite ceramic precursor,such as a silicate, alumina, an organo-metallic, or calcium, forexample. Those skilled in the art will appreciate that the ferritematerials may also be coated with other materials that decompose tonative oxides in fairly thin layers.

Coating the ferrite feed materials may be particularly advantageous whenusing the ferrite powder to form carrier beads. More specifically,coating the ferrite feed materials advantageously enhances resistivityfor carrier beads. Coating the ferrite feed materials may alsoadvantageously enhance other surface properties in applications wherespecial surfaces may be required, such as sensors and catalysts, forexample.

Coating may be a pretreatment of “virgin” ferrite feed materials, i.e.,ferrite feed materials that have not yet been exposed to the combustiongases. Coating may also be a post treatment of ferrite feed materialsthat have been exposed to combustion gases to thereby create new ferritefeed material of coated ferrite particles. Coating the ferrite feedmaterials may advantageously enhance resistivity, breakdown voltage,increase carrier life, or create novel catalysts.

At Block 48, the ferrite feed materials are exposed to the LVOF flamespray, as described above. The ferrite powder may be sorted at Block 50,and formed into a ferrite article at Block 52 before stopping at Block54.

Turning now additionally to the flow chart 60 of FIG. 3, another methodfor making ferrite powder is now described. Metal oxides are spray driedat Block 64, and exposed to the LVOF heated gas at Block 66. The ferritepowder is sorted at Block 68, and formed into a ferrite article at Block70 before stopping at Block 72.

Referring now additionally to the flow chart 80 of FIG. 4, yet anothermethod for making ferrite powder is described. From the start (Block 82)waste material is heated in a reactor while supplying oxygen at Block 84to provide metal oxide. The metal oxide is exposed to the LVOF flamespray at Block 86. The ferrite powder is sorted at Block 88, and formedinto a ferrite article at Block 90 before stopping at Block 92.

Referring additionally to FIG. 5, another aspect of the presentinvention is directed to an apparatus 100 for making ferrite powder, andis now described. The apparatus 100 illustratively comprises a feeder102 for ferrite feed materials in a form of particles, such as havingdifferent sizes and irregular shapes. As described above, the ferritefeed materials may, for example, comprise nickel ferrite, manganeseferrite, magnesium ferrite, zinc ferrite, or any combination thereof, asunderstood by those skilled in the art.

The apparatus 100 also illustratively comprises an LVOF flame spraysystem 106 for exposing the ferrite feed materials to the LVOF flamespray, such as to provide a more spherical shape to irregularly shapedparticles to thereby make the ferrite powder. The combustion andoxidation gasses are illustratively supplied to the LVOF system 106 fromcombustion and oxidation gas supplies 105. A valve (not shown) may bepositioned between the gas supplies 105 and the LVOF system 106 toregulate the flow of gasses as will be appreciated by those skilled inthe art. The LVOF flame spray system 106 may include a flame spray gunand associated equipment as available from Sulzer Metco (US) Inc. ofWestbury, N.Y.

The feeder 102 illustratively feeds the ferrite feed material into theLVOF flame spray system 106. The ferrite feed material is exposed to thecombustion gasses in the LVOF system 106.

The apparatus 100 further illustratively comprises a controller 108 forcontrolling the feeder 102 and the LVOF combustion system 106 to makethe ferrite powder to have a predetermined phase ratio, surfacemorphology, density, magnetic moment, and/or volume electricalresistivity while maximizing yield and minimizing oxidation residue. Thecontroller 108 may control the feed rate of the ferrite feed materialfrom the feeder 102, the exposure time of the ferrite feed material tocombustion in the LVOF system 106, and/or the temperature of the ferritefeed materials when being exposed to the heated gas. As indicated above,the rate of heat transfer from the hot gas may be controlled to therebycontrol the temperature of the ferrite feed material. As also indicatedabove, the volume of combustion gas, as well as the types of combustionand oxidation gasses used during LVOF exposure may also be controlled.

The apparatus 100 also illustratively comprises an optional secondfeeder 110 to supply at least one additional material to the ferritefeed material. As noted above, the at least one additional material may,for example, comprise oxygen, hydrogen, helium, inert gas, air, or anyother material as understood by those skilled in the art. The controller108 is illustratively connected to the second feeder 110 to therebycontrol variables, such as feed rate, for example, of the additionalmaterial. The apparatus 100 also illustratively includes a carrier gassupply 101 for supplying carrier gas into the stream carrying theadditional material from the second feeder 110. The carrier gas supply101 is illustratively connected to the controller 108.

The apparatus 100 further illustratively comprises a collector 111downstream from the LVOF system 106 for collecting ferrite powder, and asorter 112 downstream from the collector for sorting the ferrite powderto have particle sizes within a predetermined range. Those skilled inthe art will understand that the predetermined range is dependent uponthe article that is formed using the ferrite powder. The apparatus 100also illustratively comprises a former 114 for forming the ferritepowder into a ferrite article, such as carrier beads, an inert anode, ora body of an inductor, for example.

The additional material may also illustratively be supplied from thesecond feeder 110 to a feed stream 119 from the feeder 102 to the LVOFsystem 106 (indicated by dashed arrow 117). The carrier gas supply 101illustratively supplies carrier gas to the stream 117 carrying theadditional material to the feed stream 119.

The ferrite powder may be recycled into the LVOF system 106, asindicated by the stream of dashed arrow 121. In such a case, the ferritepowder material is taken from the sorter 112 and sent to a coater 125.The coater 125 is illustratively connected to the controller 108. Theadditional material from the second feeder 110 is also added to thecoater 125. Accordingly, the recycled ferrite powder and additionalmaterial from the second feeder 110 is supplied to the LVOF system 106.

The additional material may also be added from the second feeder 110 tothe ferrite powder after it has been passed through the sorter 112. Theadditional material is preferably provided to another coater 126(indicated by the dashed arrow 122). The ferrite powder is alsoillustratively supplied to the coater 126 from the sorter 112 (asindicated by the dashed arrow 123). Other additional material may alsobe added to the coater 126 from a third feeder 130, and may includemetal salt solution, organo-metallic, inorganic coating, or any othermaterial as understood by those skilled in the art. The third feeder 130and the coater 126 are illustratively connected to the controller 108,and carrier gas from the carrier gas supply 101 is illustrativelysupplied to the feed stream 122 from the second feeder 110.

The method and apparatus 100 described above may be used to producesmooth, spherical ferrite powder from previously sintered feedstock.More specifically, the previously sintered feedstock may have a largerthan desirable average particle diameter. The particles may be producedby first grinding the feed to a desired average diameter, classifyingthe feed to remove unwanted particles, then passing the feed through thegas heated by the LVOF flame spray process. Classifying the feed toremove unwanted particles is preferable, as exposure to the hot gas maynot alter particle diameter.

Table 1 (FIG. 6) presents a summary of the trials that were performedwhen evaluating N₂ Plasma, Argon Plasma, and LVOF processes. Theequipment used to produce the nitrogen plasma sprayed product was aSulzer Metco externally injected 9 MB gun while the argon plasma sprayedproduct was produced using an internally injected Praxair SG100 plasmagun. The LVOF system was a Sulzer Metco 6P-II gun using acetylene-oxygencombustion gases.

Table 1 illustrates that LVOF produces the best characteristics of anyof the other methods described herein. One key benefit is minimaloxidation residue (that forms small particles of the size of smoke) withmaximum bulk density and true density. The other benefit is that itproduces a very low amount of total defective powder including unmeltedpowder and miss-shaped or irregular particles. For an application suchas a carrier bead, ideally the total defective particles will be below1% with zero being optimal. It is also significant that the LVOFmaterial, as sprayed without any further refining, has a lower totalpercent defective than the finished product with any plasma method todate. It has also been seen that the ratio of the size of powderproduced in the 90^(th) percentile of mass to the size of powder in the10^(th) percentile of mass (d₉₀/d₁₀ ratio) is not a processcharacteristic of either Plasma or LVOF. The parameters that producegood material with the LVOF method have broad ranges, compared to plasmamethods, that allow good product to be made. This can also be seen bylooking at the differences in the control parameters of Table 1. It wasseen with the plasma processes that minor changes often resulted in theproduction of material that was out of specification for one of the keycharacteristics that were mentioned before.

The presence of oxidation residue, or particles of ceramic on the sizeof smoke particles causes several undesirable changes in properties. Itincreases surface area and decreases good flow properties of the powder,making it difficult to handle. Oxidation residue or (smoke particles)are difficult to directly quantify, therefore these properties, surfacearea, bulk density and flow rate are used to indicate its presence. Thepresence of this dust is most pronounced after the plasma or LVOFprocessing before subsequent steps of the methods.

Table 2 (FIG. 7) shows the surface area, yield and other properties ofthe powders after plasma or LVOF processing. The argon plasma methodmakes a very high surface area (a consequence of the hot plasma)followed by the nitrogen plasma process. Material made with the LVOFprocess shows the desirable lowest surface area. This is also manifestedby the lower flow rate of the argon plasma processed powder compared tothe powder processed by the LVOF. The final indicator of less “dust” isthe higher yield. While the yield of the nitrogen plasma is similar toLVOF, nitrogen plasma processing produces much more defective powder andpowder with lower true density.

Table 3 (FIG. 8) compares the energy costs that show that producingpowder via LVOF is at least $0.18 per pound less than a plasma process.This cost advantage is even more significant when the recurring cost ofsupplies, equipment and repair are considered. The plasma process usedan electric arc and complicated electrical insulation and cooling ofanodes and cathodes is required. Electrode life is relatively short forthe plasma gun. The yield is higher with the LVOF process, 13% betterwith LVOF on average; therefore similar savings in raw materials costsare gained.

Referring now additionally to the photomicrographs of FIGS. 9-12 furtherfeatures are now described as relating to the effect of thermal sprayprocesses and “dust” formation. FIG. 9 shows a plasma sprayed particleusing nitrogen. FIG. 10 shows a plasma sprayed particle using argon. Incontrast, FIG. 11 shows a particle made using the LVOF process. Althoughthe size of the particles is near the limit of resolution of thescanning electron microscope, FIG. 9 shows prominent occurrence of verysmall powder on the surface of the large particle (see arrow “a”).Another consequence of the nitrogen plasma spray is a large amount ofother residue noted by the arrow “b.” Note the relatively smooth surfaceof the nitrogen-processed plasma sprayed particle. FIG. 10 shows acomparable particle made using argon gas in the plasma process. Note theparticle is quite irregular and has a lot of the small “dust” particleson it. This “dust” contributes to poor flow properties. Also note thatthe argon-processed plasma particle is not very spherical and does nothave a very smooth surface.

FIG. 11 shows a comparable powder made using the LVOF process. Note thatthe surface is relatively clean compared to the plasma processed powderparticles. Also note the highly spherical and smooth particle.

FIG. 12 shows an example of the unmelted particles seen at higherfrequency in the nitrogen plasma spray processed powders.

One of the benefits of using the LVOF spray process is that it producespowder with better technical properties and a lower energy process thanplasma spraying, for example. The LVOF spray process introduces powderaxially to the temperature source to allow a more uniform temperaturedistribution than processes that inject powder perpendicular to the flowof gasses, for example. The advantage here is that the lower temperaturemay prevent the ferrite from oxidizing and vaporizing to formultra-small particles on the order of size of “smoke.” Such small powdermay cause particularly poor flow properties.

In contrast, the plasma spray process may require a very limited balancebetween the gas flow rate, temperature, and injection pressure toachieve spheroidization, but avoid excessive heat transfer thatvaporizes part of the feed material. The LVOF method is more forgiving;it has a larger window of process variables that produces ferrite powderwith high bulk and true density. In addition, the lower gas velocity mayresult in less particle distortion giving less total defective(non-spherical) powder.

LVOF processing develops heat through combustion gases, while plasmaprocess spray guns use an electric arc to ionize a primary and secondarygas to form a hot plasma. As will be appreciated by those skilled in theart, the most common fuels used in LVOF are oxygen, hydrogen, MAPP(methylacetylene and propadiene), and acetylene. MAPP, hydrogen oracetylene are not typically run simultaneously. When LVOF is combinedwith a co-axial feed method to the gas flow it may allow a moreconsistent introduction of the particles to the heat source, but doesnot vaporize them as frequently occurs in axial injection inside theplasma gun. This improved consistency may reduce the total irregularparticles (hollow and miss-shaped) to achieve powder with the same orbetter density.

The typical flame temperature for LVOF is ˜3000° C. compared to 15,000°C. for a true plasma temperature. Accordingly, the preferred temperaturemay be desirably less than about 5000° C. and more preferably about3000° C. Because the gas velocity and temperature are lower, the flamevelocity of LVOF is 600-700 ft/sec compared to 1500-1800 ft/sec forplasma and the flame is almost twice as long with LVOF. In other words,the flame velocity may be desirably less than about 1000 ft/sec, andmore preferably in the range of 600-700 ft/sec. This difference inlength results in a residence time almost five times longer than plasmaspraying. The exact values for the flame temperature and velocity aredependant on the fuels being used and their proportions as will also beappreciated by those skilled in the art.

LVOF may also be useful to conventional “press-and-sinter” ceramicprocesses that are used to make dense ferrite and other ceramic bodies.It may be considered an economical and efficient replacement of therotary kiln used to pre-sinter ferrite, or other ceramic powders afterspray drying. It may be particularly useful to make magnetite powderdirectly. Hematite material run at various feed rates may produce powder10% denser than calcined product with an EMU that is consistent formaterial that has been transformed to Magnetite. Since pre-sintering ina rotary kiln typically requires significant startup costs, LVOF can beused advantageously for short or custom runs of material.

Referring now additionally to FIGS. 13A-13C, further aspects of the LVOFmethods are now described. The LVOF process is outlined in the leftcolumn (FIG. 13A) as compared to the conventional approach in the centercolumn (FIG. 13B). The LVOF green process is in the right column (FIG.13C). The flow of material according to the steps indicated on the flowcharts of FIGS. 13A-13C are as follows.

At Block 152 raw materials are mixed and pelletized with water toproduce calciner feed material. The materials are reacted with high heatin a rotary furnace to begin spinel formation and reduce variations infinal firing at the calcine Block 154. At the ball mill Block 156 metaloxides are combined and milled in an aqueous suspension to achieve thedesired particle size. The slurry is checked to verify composition andadditions are used to achieve the desired rheology prior to the slurrybeing spray dried at Block 158 to achieve desired particle size. At thegreen screen Block 160 the spray dried powder is sorted/screened toachieve the correct particle size distribution prior to preliminarydensification.

A bisque operation is performed at the Block 161 (FIG. 13A); however, inthe LVOF process only, sorted powder is partially reacted in an airenvironment to improve particle integrity and eliminate additives.

A sinter is performed at Block 162 (FIG. 13B) wherein materials thatfollow the standard flow are sintered to final density at hightemperature. At Block 164 material from the kiln is deagglomerated toeliminate the large chunks of material that form in the kiln. Thepurpose here is to eliminate satellites and agglomerates so that firedscreening can be done efficiently.

The powder is screened at Block 166 to achieve the target particle sizefor the LVOF process (Block 167, FIGS. 13A, 130) or heat treatment(Block 168) in the case of the standard process (FIG. 13B). The LVOF(Block 167) may be considered a spheroidization process—it has the samebenefits as plasma spraying on homogeneity but the operating andequipment costs are significantly reduced. LVOF processing also producesmaterial that has a more uniform density.

The heat treatment at Block 168 may be a combination of annealingtreatments that are designed to produce a final product that meets themagnetic and resistivity specifications. At Block 170 a sinteredscreening step is performed to produce the target particle size rangefor the customer application. The key outputs from this are d₅₀,d₉₀/d₁₀, and a desired percentage less than 20 μm, for example.

Classification (Block 172) is utilized to remove additional undesiredmaterial. This typically removes higher surface area particles such asincluding unmelted particles and particles that are less than 10 μm.Prior to shipment (Block 174) all characteristics are tested andverified.

In the LVOF standard process (FIG. 13A), the material after the bisqueBlock 161 is still comprised of metallic oxides in their natural form,the sintering process is only partially completed to maximize screeningyields and to maintain the tightest possible distribution. If thesintering is performed more completely, or in different atmospheres theeffect is powder that is agglomerated and difficult to break up andscreen efficiently. The LVOF feed materials may not be ferrite, thepartial sintering is not a means to produce spinel, the atmosphere andthe temperature are controlled in a manner that allows densification tooccur to the point that strength is increased but there is not anyreduction of the metal oxide components.

In the case of the LVOF green process (FIG. 13C) the feed materials aremixed, unreacted oxide powders. The integrity of these particles ismaintained through the use of binders that are added in the spray dryingprocess.

Although the present LVOF thermal spray process described herein isadvantageous for producing carrier bead, it may be used for producingother products as well, and as will be appreciated by those skilled inthe art. In addition, to producing environmentally friendly materials,such as MnFerrite, MnMgFerrite, and Magnetite, the process may also beused to produce CuZn and NiZn materials, for example. The LVOF processcan eliminate the sintering step in the production of powders. The LVOFprocess offers an advantage over the plasma spray approach because itrecovers the off-size product and it provides flexibility to processdifferent materials and formulations, for example.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is to be understood that the invention is not to belimited to the specific embodiments disclosed, and that othermodifications and embodiments are intended to be included within thescope of the appended claims.

1. A method for making ferrite powder comprising: providing ferrite feedmaterials in a form of particles; and exposing ferrite feed materials inthe form of particles to a low velocity oxygen-fueled (LVOF) flame sprayto thereby make the ferrite powder.
 2. A method according to claim 1wherein exposing comprises axially introducing the ferrite feedmaterials in the form of particles to the LVOF flame spray.
 3. A methodaccording to claim 1 wherein the LVOF flame spray operates at atemperature of less than about 5,000° C.
 4. A method according to claim1 wherein the LVOF flame spray operates at a flame velocity of less thanabout 1,000 feet per second.
 5. A method according to claim 1 whereinthe ferrite feed materials prior to exposing comprise irregularly shapedparticles; and wherein exposing produces more spherically shapedparticles from the irregularly shaped particles.
 6. A method accordingto claim 1 further comprising controlling at least one of providing andexposing to make the ferrite powder to have at least one of apredetermined phase ratio, surface morphology, density, magnetic moment,and volume electrical resistivity.
 7. A method according to claim 6wherein controlling comprises controlling at least one of a feed rate,an exposure time, and a temperature of the ferrite feed materials duringthe exposing.
 8. A method according to claim 6 wherein controllingcomprises controlling a composition of the ferrite feed materials.
 9. Amethod according to claim 8 wherein controlling comprises controllablysupplying at least one additional material to the ferrite feed materialsduring the exposing.
 10. A method according to claim 9 wherein the atleast one additional material comprises at least one of oxygen,hydrogen, an inert gas, and calcined ferrite feed materials.
 11. Amethod according to claim 9 wherein controllably supplying comprisescoating the ferrite feed materials with at least one of a silicate,alumina, and an organo-metallic.
 12. A method according to claim 1wherein the ferrite feed materials comprise at least one of nickelferrite particles, manganese ferrite particles, magnesium ferriteparticles, strontium ferrite particles, and zinc ferrite particles. 13.A method according to claim 1 wherein the ferrite feed materialscomprise metal oxides.
 14. A method according to claim 1 furthercomprising sorting the ferrite powder to have particle sizes within apredetermined range.
 15. A method for making a ferrite articlecomprising: providing ferrite feed materials in a form of particles;exposing ferrite feed materials in the form of particles to a lowvelocity oxygen-fueled (LVOF) flame spray to thereby make a ferritepowder; and forming the ferrite powder into a ferrite article.
 16. Amethod according to claim 15 wherein forming comprises forming theferrite powder into carrier beads.
 17. A method according to claim 15wherein forming comprises forming the ferrite powder into an inertanode.
 18. A method according to claim 15 wherein forming comprisesforming the ferrite powder into a body of an inductor.
 19. A methodaccording to claim 15 wherein exposing comprises axially introducing theferrite feed materials in the form of particles to the LVOF flame spray.20. A method according to claim 15 wherein the LVOF flame spray operatesat a temperature of less than about 5,000° C.
 21. A method according toclaim 15 wherein the LVOF flame spray operates at a flame velocity ofless than about 1,000 feet per second.
 22. A method according to claim15 wherein the ferrite feed materials prior to exposing compriseirregularly shaped particles; and wherein exposing produces morespherically shaped particles from the irregularly shaped particles. 23.A method according to claim 15 further comprising controlling at leastone of providing and exposing to make the ferrite powder to have atleast one of a predetermined phase ratio, surface morphology, density,magnetic moment, and volume electrical resistivity.
 24. A methodaccording to claim 23 wherein controlling comprises controlling at leastone of a feed rate, an exposure time, and a temperature of the ferritefeed materials during the exposing.
 25. A method according to claim 23wherein controlling comprises controlling a composition of the ferritefeed materials.
 26. A method according to claim 25 wherein controllingcomprises controllably supplying at least one additional material to theferrite feed materials during the exposing.
 27. A method according toclaim 15 wherein the ferrite feed materials comprise at least one ofnickel ferrite particles, manganese ferrite particles, magnesium ferriteparticles, strontium ferrite particles, and zinc ferrite particles. 28.A method according to claim 15 wherein the ferrite feed materialscomprise metal oxides.
 29. An apparatus for making ferrite powdercomprising: a feeder for ferrite feed materials in a form of particles;and a low velocity oxygen-fueled (LVOF) flame spray system for exposingthe ferrite feed materials to an LVOF flame spray to thereby make theferrite powder.
 30. An apparatus according to claim 29 wherein said LVOFflame spray system axially introduces the ferrite feed materials in theform of particles to the LVOF flame spray.
 31. An apparatus according toclaim 29 wherein said LVOF flame spray system generates the LVOF flamespray at a temperature of less than about 5,000° C.
 32. An apparatusaccording to claim 29 wherein said LVOF flame spray system generates theLVOF flame spray at a flame velocity of less than about 1,000 feet persecond.
 33. An apparatus according to claim 29 further comprising acontroller for controlling at least one of said feeder and said LVOFflame spray system to make the ferrite powder to have at least one of apredetermined phase ratio, surface morphology, density, magnetic moment,and volume electrical resistivity.
 34. An apparatus according to claim33 wherein said controller controls at least one of a feed rate, anexposure time, and a temperature of the ferrite feed materials duringthe exposing.
 35. An apparatus according to claim 33 wherein saidcontroller controls a composition of the ferrite feed materials.
 36. Anapparatus according to claim 29 further comprising a second feeder tosupply at least one additional material to the ferrite feed materials.37. An apparatus according to claim 29 wherein the ferrite feedmaterials comprise at least one of nickel ferrite particles, manganeseferrite particles, magnesium ferrite particles, strontium ferriteparticles, and zinc ferrite particles.